This application focuses on three key steps in Salmonella infection; how the bacteria adhere to cells, kill or disarm phagocytic cells, and then replicate in only specific tissues of the body. The mechanism by which Salmonella colonizes the small intestine is not known. The investigator has identified a new class of pili that adhere to M cells. A mutant strain missing this pilus is partially attenuated for virulence (five fold) suggesting alternative adherence factors (Baumler et al., 1995). The investigators will use several approaches to identify these other adherence factors and study the surface components of the cell to which they bind. Once the bacteria are within the lymphatic system or the spleen and liver they pursue a lifestyle that is partly extracellular. Salmonella bacteria kill and lyse the macrophage after about 24 hours of infection in vitro. The goal of Salmonella may be to reach and disarm the macrophage and use the host cells nutrients for its own growth. The investigators have selected transposon mutants that grow in macrophages at the same rate as the parent strain but without killing. The first two independent mutants that were analyzed are located about 300 bp apart in ompR, a member of the two component regulator family. These mutants are totally avirulent. Several other mutants were identified in the same selection but have not yet been characterized. They will clone and sequence the cognate genes and determine their role in pathogenesis in the mouse. Finally, Salmonella lives within a limited number of cell types within the host. In part, this tissue tropism is defined by nutrition, e.g., none of the known mechanisms to take up Fe(III) or Fe(II) appears to play any role in virulence. We have identified a new Salmonella iron uptake mechanism that is essential for virulence.